The present invention is in the field of photocatalysts for the degradation of organic pollutants. The present invention provides a novel semiconductor photocatalyst for use in such degradation, as well as a process for preparing such photocatalyst.
Semiconductor photocatalysis for water and air purification has evoked considerable interest during the last decade. The application of such catalysts for the degradation of contaminants has been used successfully for the degradation of various organic compounds such as alkanes, aliphatic and aromatic carboxylic acids, aliphatic and aromatic halogenated compounds, alkenes, phenols, amines, surfactants, pesticides, heterocyclic compounds, as well as for the reductive deposition of heavy metals.
Titanium dioxide (TiO2) is a chemically inert compound, stable under illumination conditions, relatively inexpensive and has proven to be an especially useful semiconductor for environmental applications (Schwitzbegel, J. et al., J. Phys. Chem., 99 (1995), 5633). Excitation of TiO2 by light energy leads to the formation of an electron-hole pair and in the presence of a suitable scavenger or surface states, the electron or hole are trapped, recombination is prevented and subsequent redox reactions occur. The photocatalytic degradation of organic materials by TiO2 particles in aqueous media and in the presence of oxygen, is initiated by the formation of free hydroxide radicals (OH) which are capable to degrade organic compounds into water and carbon dioxide.
Effective degradation of organic pollutants is accomplished when the pollutant is preconcentrated at the semiconductor surface. There are known in the art several methods for enhancing interfacial electron-transfer reactions of TiO2, wherein the properties of the TiO2 particles have been modified by selective surface treatments such as surface chelation (J. Moser et al., Langmuir, 7 (1991), 3012), surface derivatization (Hong, A. P., et al., J. Phys. Chem., 91 (1987), 6245) and selective doping of the crystalline matrix (Lee, W. et al., Mater. Res. Bull., 28 (1993), 1127).
TiO2 can be obtained by a sol-gel reaction, upon heating a gel containing it to a temperature of above 450xc2x0 C. By dispersing or dissolving substances in such a matrix, a solid TiO2 with homogeneously mixed substances may eventually be obtained. Furthermore, a TiO2 solid prepared by the sol-gel method is characterized by a very large specific surface area and high pore volume, rendering this solid an excellent absorbent of various, molecules or ions which bind to its surface. The large specific surface area makes the TiO2 attractive for catalytic applications.
Commercial products coated with TiO2 having photoelectrochemically active surfaces have now entered the market. For example, the Japanese company Toto is marketing tiles for operating rooms and other hospital applications. Such tiles are also being used in rest rooms where they breakdown organic compounds deposited on the tiles and thus reduce the amount of microorganisms which grow on said organic compounds and thereby eliminate malodors. Another use of TiO2 coated products is for street lighting appliances, from which organic pollutants emitted from vehicles have to be removed.
The adsorption of organic compounds onto the semiconductor surface is a critical step in the photodegradation process. Therefore, the preconcentration of the organic pollutant at the semiconductor surface is an especially important feature. It has been found, in accordance with the present invention, that such preconcentration can be enhanced by incorporating lanthanide ions such as europium or praseodymium ions, in a titanium dioxide matrix, e.g. by means of the sol-gel method mentioned above. The lanthanide ions incorporated in such a titanium dioxide matrix are capable of forming Lewis acid-base complexes with organic moieties such as double bonds, acids, amines, aldehydes, esters, ethers, thiols, alcohols, etc., and, hence, impart to the titanium dioxide photocatalyst enhanced activity for the degradation of organic compounds bearing such moieties.
It is an object of this invention to provide novel TiO2 semiconductor photocatalysts (a photocatalyst being a catalyst induced to perform a catalytic activity by light) doped with oxides of lanthanide metals, which exhibit enhanced activities for the degradation of organic compounds and particularly organic pollutants.
It is a further object of the present invention to provide a process for the preparation of such photocatalysts.
The present invention provides a photocatalyst comprising TiO2 doped with at least one lanthanide oxide. Preferably the molar ratio Ti:M being from about 100:0.5 to about. 100:5. At times, the lanthanide oxides may be incorporated as hydrates.
All elements with atomic numbers from 58 to 71 are known as lanthanide series, including Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu. These elements are inner-transition elements. i.e. their electron configuration feature the filling of 4f orbitals and are capable to form complexes with electron donors.
Preferred lanthanide oxides for use in accordance with the invention are europium oxide, praseodymium oxide and ytterbium oxide. At times other metals may be incorporated into the photocatalyst, such as sodium, magnesium, iron, cobalt or palladium. Such other metals may be incorporated as ions, oxides and other metal containing compounds as complexes. Incorporation of such other metals should be at a level which will not seduce the catalytic activity of the photocatalyst.
As shown hereinbelow, TiO2 photocatalyst doped with a lanthanide oxide, according to the present invention, exhibit enhanced activity, as compared to undoped TiO2 photocatalyst, in degradation of functionalized organic compounds. The term xe2x80x9cfunctionalized organic compoundxe2x80x9d denotes any compound having a functional group which may be a donor of electrons such as a double bond, carboxylic group, carbonyl group, aldehyde group, amine group, thiol, hydroxyl, ether, heterocyclic organic substrates, phosphate, phosphonate, etc. Organic compounds with functionalized groups may include a variety of noxious substances such as herbicides, intermediates used for the preparation of herbicides, various organic compounds used in industrial synthesis, pollutants, e.g. such emitted by vehicles or industrial combustors or power generators, airborne compounds with a malodor, chemical warfare gases such as nerve gas, mustard. gas, etc.
A specific example of functionalized organic compounds which may be degraded by the photocatalyst of the invention are a variety of aromatic compounds, e.g. such present in herbicides which include, inter alia, p-chlorophenoxyacetic acid, salicylic acid, trans-cinnamic acid, aniline and p-nitrobenzoic acid. Other specific examples include a variety of oily pollutants, e.g. in water.
By another aspect, the present invention provides a process for preparation of the above TiO2-based photocatalyst. The process of the invention comprises: mixing a titanium (Ti) containing compound with a lanthanide metal (M) containing compound, to form a gel, the molar ratio Ti:M being from about 100:0.5 to about 100:5; drying the gel thus obtained; and subjecting it to calcination.
The titanium containing compound is typically isopropyl titanoate or titanium chloride. The lanthanide compound may either be lanthanide oxide or may be a precursor compound from which the lanthanide oxide may be obtained. A lanthanide precursor compound may for example be acetyloxy lanthanide or lanthanide nitrate.
The sol is typically prepared in an alcohol solution, the solvent being a lower alkanol. The gel may be formed by adding a catalytic amount of a strong acid to the sol.
The heat treatment may include a first drying step to cause evaporation of the alcohol and then a second calcination step which may be at a temperature of at least about 600xc2x0 K at times above about 800xc2x0 K.
The photocatalyst of the invention may be used in different forms and applied on different matrices for use in different media. Depending on its planned use, it may be formed into a powder, it may be formed into solid particles, it may be formed as a coating of a solid surface, it may be impregnated into an absorbent substrate, it may be incorporated into a liquid medium intended for application onto a surface (e.g. paint formulation), etc.
One example of use of the photocatalyst of the invention is to apply it on a surface on which various undesired organic materials are deposited so as to impart self-cleaning properties to such surfaces. For example, it may be applied on substrates such as floor or wall tiles for degrading various organic substances which may otherwise may become deposited on the surface and form a substrate for a growth of microorganisms which may give rise to health hazards and malodors (e.g. in restrooms). In another example, it may be applied onto glass or other translucent or transparent surfaces, e.g. those in street lamps, in order to degrade various substances which are emitted from vehicles"" engines and which thus become deposited on to these surfaces. The application of the photocatalyst of the invention on to such surfaces may be by applying the gel on to the surface (e.g. by immersion into the gel, by spraying or by other known coating techniques) and then heat treating the surface to dry the gel and cause it to calcinate. Additionally, the photocatalyst may be applied as a powder within a liquid formulation (e.g. a formulation comprising the photocatalyst particles and binders such as a paint formulation).
By another example, the photocatalyst of the invention is formed into air-pockets containing particles or coated or impregnated into such particles, which particles have the ability to float on water. Such particles are useful, for example, in degrading oil spills in water, in treatment of industrial effluents, etc. A particular example, may be the use of the photocatalyst of the invention applied as a coat of hollow glass spheres.
By another example, the photocatalyst of the invention may be impregnated into an absorbent material, e.g. a filter used in air conditioning systems, a filter used for filtering aqueous media, and others. In the air conditioning systems, impregnation of the filter with the photocatalyst of the invention will degrade organic substrates which are absorbed by the filter, which may otherwise serve as a growth medium for microorganisms.
By another example, the photocatalyst of the invention may be used to clean air or another medium from chemical warfare gases. In such use the photocatalyst may be applied to walls of shelter, incorporated in filters of shelters or gas masks, applied onto protective clothes, etc.
Other uses of photocatalysts of the invention may be in degradation of leftover pesticides, e.g. within the framework of a post-harvest treatment, cleaning water reservoirs from toxic and hazardous organic wastes, use in reactors and in purification plants for removal of organic waste from water, and any other applications.
In accordance with the invention there is further provided a method for the degradation of organic materials in an aqueous or gaseous environment, comprising contacting said medium with the photocatalyst of the invention.
It should be noted that in order to perform its catalytic activity, the photocatalyst of the invention should be exposed to light, which may be sun-light or artificial light, particularly U.V. light. In some applications the plotocatalyst of the invention may be included in a filtration system together with a light source, e.g. a U.V. light source.